It is known that certain types of obstructions in arteries may be due, in part, to arteriosclerotic plaques. These plaques typically result from the proliferation of smooth muscle cells and associated fibrous tissue which invades the wall and lining cells (intima) of the artery. While the reasons for the smooth muscle and fibrous cell proliferation in the arterial walls is not completely understood, this proliferation is not generally considered to be neoplastic in origin, in spite of the fact that regeneration of plaque can take place very rapidly, sometimes within a few months of total removal by coronary endarterectomy.
Management of arteriosclerotic stenosis by balloon angioplasty is a common treatment method; however, the effectiveness of such treatment is limited by restenosis. Restenosis occurs in about 30% to 50% of patients having undergone angioplasty. Fibrocellular intimal hyperplasia is a main cause of such restenosis which arises from proliferation of smooth muscle cells in the intimal layer.
Porphyrins are a large class of typically red or purple fluorescent crystalline pigments, with natural or synthetic origin, having in common a substituted aromatic macrocyclic ring consisting of four pyrrole-type residues, linked together by four methine bridging groups. It is recognized that smooth muscle cells which proliferate in arteries have a distinct affinity for various porphyrin compounds such as HPD, photofrin, photofrin II, and a long list of other porphyrin compounds. A proliferating smooth muscle cell will take up such porphyrin compounds much in the same manner as cells which are either dysplastic or overtly malignant. Because these cells become sensitized by these porphyrin compounds, they are capable of responding to both photo-detection and photo-destruction when proper frequencies of light are administered. Use of this “photo-dynamic” therapy in the management of angioplasty restenosis in patients is described in Photodynamic Therapy of Normal and Balloon Injured Rat Carotid Arteries Using 5-Amino-Levulinic Acid, Circulation, 91(2):417-25 (1995), incorporated herein by this reference in its entirety for disclosing basic procedures for photodynamic therapy of arteriosclerotic abnormalities.
Although many physicians and researchers are familiar with photodynamic therapy and many have used such procedures in the laboratory, few have become advocates of the therapy because of the severe limitations imposed by the use of porphyrins which utilize light frequencies that do not penetrate, and are therefore impossible to deliver to any significant depth in tissue. The light frequencies required for photo detection generally range between 380-420 nm, and the resulting fluorescence is typically in the range of 635 nm. Because of these wavelengths, penetration of the light source is restricted to tissue of minimal depth in the body. Accordingly, without surgical intervention, phototherapy is not capable of effectively reaching arteriosclerotic lesions.
As also understood by those skilled in the art, photodynamic therapy has been used for treatment of various cancers. Examples of references which disclose use of photodynamic therapy for treatment of cancer include the U.S. Pat. Nos. 5,087,636 and 5,211,938.
Another significant, well known method for treatment of arteriosclerotic abnormalities includes localized intercoronary radiation therapy. This therapy is reviewed in Localized Intercoronary Gamma Radiation Therapy to Inhibit the Recurrence of Restenosis after Stenting, and Endoluminal Beta Radiation Therapy for the Prevention of Coronary Restenosis after Balloon Angioplasty, The New England Journal of Medicine, 344(4)243-56 (2001). The studies reported therein indicate significantly lowered rates of clinical and angiographic restenosis following radiation therapy.
There are also a number of references which further disclose radiation therapy for arteriosclerotic abnormalities including U.S. Pat. Nos. 6,422,989; 6,422,988; 6,935,016; 6,387,350; 6,358,989; and 6,235,767.
Finally, there is a known treatment for cancer which utilizes metaloporphyrins to deliver site selective radiation therapy. More specifically, U.S. Pat. No. 5,391,547 discloses a method for using porphyrins to detect lung cancer by the use of tetra-aryl porphyrins. The porphyrins are used as fluorescent tracers for cancers of the lung. The porphyrins are complexed with 64Cu or 67Cu. Thus, the complex can be used as radiotracers as well. The 67Cu provides a source of beta radiation for selective destruction of lung malignancies as well as gamma radiation useful for image analysis, as by a single photon emission computed tomography (SPECT). The 64Cu as a positron emitter, may be used for radiotracing wherein positron emission tomography (PET) techniques can be used to locate the malignant tissue.
While the aforementioned radiation treatments for arteriosclerotic abnormalities have shown some promise, one significant drawback to known procedures is the inability to effectively localize the radio compounds in the targeted tissue. Furthermore, such radiation treatment is typically done after there has already been an interventional procedure conducted, such as balloon angioplasty or stent emplacement. Thus, such radiation is primarily used as a follow-up treatment and not an initial treatment of arteriosclerotic abnormalities.
While photodynamic therapy also has been proven to be effective in prevention of arteriosclerosis, photodynamic therapy in practice is extremely difficult to incorporate because an illuminating catheter must be delivered to the damaged arterial locations and even after the catheter has reached the site to be treated, normal blood flow through the arteries further complicates the ability to deliver an effective intensity of light to the targeted tissue.
Therefore, while photodynamic therapy and radiation treatment can potentially be effective, there is still a need for a non-interventional procedure for treatment of arteriosclerotic abnormalities which provides not only an initial screening or diagnosis, but also may be simultaneously used for actual treatment of the affected blood vessels to reduce and destroy plaque and prevent or eliminate restenosis.